Flu has become prevalent throughout New York, state Health Commissioner Nirav Shah said Thursday.

The declaration signals the widespread reach of the contagious illness and triggers a new requirement for some health workers to wear masks if they have not received flu vaccinations this season.

As of last Saturday, 45 counties — including Albany, Rensselaer, Schenectady and Saratoga — and all boroughs of New York City had laboratory-confirmed cases of influenza, according to the state. The number of confirmed cases last week totaled 284, a 54 percent spike over the previous week. On Dec. 7, the flu was still considered "local," though cases had shot up 133 percent from the previous week.

Symptoms of influenza can include the sudden onset of fever, chills, headache and muscle aches, as well as a cough or sore throat. Though common, the seasonal flu can result in serious complications for some patients, who may develop severe respiratory infections, like pneumonia, and cannot recover.

The virus poses a more serious risk for people under 2, those over 50, pregnant women and people with weakened immune systems or chronic medical conditions, according to the state Health Department.

The flu season got off to an earlier start this year compared to last, with laboratory-confirmed cases reported to the state by the second week of October.

But flu was widespread earlier last year — by late November. Last year's season turned out to be a doozy. More than 45,000 New Yorkers had laboratory-confirmed cases of the illness, a 10-fold increase over the previous year.

On Thursday, the state Health Commissioner urged New Yorkers who have not been vaccinated against the flu to do so now. The vaccine is most effective if given before exposure to the illness, and it takes two weeks for the body's immune system to ramp up a full defense against the illness. But since flu often peaks in late winter or early spring, getting the vaccine now could prove effective, Shah said.

"A flu vaccination is a safe and effective way to reduce your risk for flu and also protect the health of your family and friends," he said.

The vaccine must be given each year — it is not effective from one year to the next. In an interview earlier this week, Dr. Elizabeth Higgins of Albany Medical Center said the flu strains in this year's vaccine batches match the actual illness circulating, so it is highly effective.

Health workers who come into direct contact with patients must wear surgical masks for the first time this year if they do not get vaccinated. The measure is intended to prevent patients from getting sick from nurses, aides and others caring for them. Masks are not as effective as vaccine in preventing the transmission of flu, but should lessen the chance of spreading the illness, according to the state Health Department.

The regulation was put into effect this summer, after previous failed attempts to require health workers to get inoculated against the flu.

The Public Employees Federation, which represents nurses who work in state-run hospitals, is fighting the regulation in court. PEF President Susan Kent said last month that the union would "not allow the state to bully our members into getting a vaccination."

Among other allegations, the PEF lawsuit contends that the masks could frighten some mentally disabled patients. The masks could also become "germ-filled cesspools" for their wearers, PEF claims in court documents.

But much of the state's medical establishment, including groups representing doctors and hospitals, has supported the regulation. Many hospitals voluntarily instituted mask rules during last winter's flu outbreak.